Typically, in a single-stage heating and cooling system, the heating system includes a low-voltage operated gas valve which controls the flow of gas to the furnace; the cooling system includes a contactor having a low-voltage coil and high-voltage contacts, which contacts control energizing of the compressor; and the circulation system includes a fan relay having a low-voltage coil and high-voltage contacts, which contacts control energizing of the fan which circulates the conditioned air.
The electrical power for energizing such low-voltage operated devices is provided either by a single transformer or by two separate transformers. If the heating and cooling system is installed as a complete unit, generally a single transformer is provided. Such a single transformer has the required volt-ampere output to operate all the low-voltage operated devices. If the cooling system is installed separate from the heating system, generally an additional transformer is used.
Specifically, in a system for heating only, a fan relay is generally not provided since the fan is generally controlled directly by a thermal switch on the furnace. Therefore, it is common in a system for heating only that the only electrical load on the transformer is the gas valve. When such a heating system is used in combination with a cooling system, the electrical load increases due to the addition of the fan relay and the contactor. The existing transformer often does not have the required volt-ampere output to operate all the low-voltage operated devices, therefore, additional transformer load capacity for the cooling system is required. Often, a second independent transformer is utilized due to the increased electrical load requirements of the cooling system. Even if the first transformer has enough load capacity for heating and cooling systems, the second transformer is generally used so as to simplify the electrical wiring involved in the installation of the cooling system.
It is desirable that a low-voltage space thermostat for controlling a single-stage heating and cooling system be constructed so as to enable it to be readily usable with either the single-transformer or two-transformer power source. While use with the single-transformer power source poses no problem, a problem exists when used with the two-transformer power source. The problem is that the two transformers might be interconnected at the thermostat in such a manner so that they are out of phase with each other, whereby the voltages at the secondary windings are additive and thereby an unacceptably high value of voltage potential may exist between various nodes in the two systems. For typical transformers having a rated 24 volt RMS secondary voltage, this unacceptably high value is approximately 68 volts peak voltage.
One prior art approach to negating this problem has been to incorporate means for isolating the secondary windings of the two transformers from each other. For example, in a related art construction, typified in U.S. Pat. No. 4,049,973 to Lambert, five wiring terminals are provided in the thermostat. Two of the thermostat terminals, isolated from each other with respect to the internal circuitry of the thermostat by a multi-position system selector switch, are normally connected together at the terminals by a removable wire jumper. When the heating and cooling system uses a single transformer, the wire jumper is retained, and one end of the secondary winding of the single transformer is connected to one of the two jumper-connected terminals. The other end of the secondary winding is connected through the fan relay, gas valve, and contactor to the remaining three terminals. When the heating and cooling system uses two transformers, the wire jumper is removed, and one end of the secondary winding of the first transformer is connected to one of the two terminals previously connected by the wire jumper. Further, one end of the secondary winding of the second transformer is connected to the other of the two terminals previously connected by the wire jumper. The other end of the secondary winding of the first transformer is connected through the gas valve to one of the three remaining terminals, and the other end of the secondary winding of the second transformer is connected through the fan relay and contactor to the remaining two terminals. Since the two terminals previously connected by the wire jumper are isolated from each other, the secondary windings of the two transformers are therefore also isolated from each other.
A second approach for solving the aforementioned problem is described in U.S. Pat. No. 4,898,229 to Brown et al. Brown et al. uses an integral circuit means to detect the existence of an unacceptably high voltage potential between the two wiring terminals. If an unacceptably high voltage potential is detected, the circuit means alerts the party installing the second transformer that the two transformers are out of phase. However, utilizing this method requires the installer to reverse the connection at the terminals. If the installer ignores the alert, the high-voltage potential is still present. Further, Brown et al. interconnects the heating and cooling transformers at terminal R of FIG. 1. This interconnection is undesirable, as the National Electrical Code discourages such a connection. Applicant's invention is an alternative to Brown et al. and Lambert, in which the polarity of the transformers is not of concern, due to the use of full-wave rectifiers in the first embodiment and the isolation of the cooling system from the heating system by means of an isolation transformer for the second embodiment.